Mercury (Hg) is a global contaminant emitted to the atmosphere from both natural and anthropogenic sources, which have varied over time in response to increased industrialization and pollution control measures. The Monte Amiata Mining District (MAMD; Central Italy) hosts the 3rd largest Hg-district worldwide and an important geothermal field actively exploited for energy. Mining ceased in the 1980s but left an impressive legacy of environmental impact. The exploitation of geothermal energy started in 1959; currently, there are five active plants equipped with emission control systems (AMIS) to reduce Hg and H2S emissions; however, the efficiency of these systems is not 100% and Hg0 is still emitted from geothermal plants. Due to the Hg availability in the environment, plants are continuously exposed and absorb by resulting low-cost and efficient biological indicators. Specifically, trees store pollutants in tree-rings that have the potential for reconstructing past atmospheric Hg to complement the temporally and spatially limited data provided by the instrumental record, producing high-resolution data with absolutely dated chronologies. Here, we present a reconstruction of atmospheric Hg0 recorded in chestnut (Castanea sativa Mill.) tree-rings (1969−2020) from two different sites of the MAMD: i) site I near the former Abbadia SS mine; ii) site II near the main geothermal power plant. At each site three trees were sampled. A tree from the Appennino Pistoiese (150 km far from MAMD) (reference area) was analyzed for background value. Total Hg concentrations in the tree-rings were quantified with a Milestone tri-cell Direct Mercury Analyzer (DMA-80). The tree-rings in the MAMD provided a temporal reconstruction of Hg exposure at annual resolution. In the mining period Hg concentrations in trees from Abbadia SS varied between 244 and 50 ng/g, whereas in the post-production period after mine closure in the early 1980s, Hg showed a distinct decrease (between 113 and 12 ng/g). The trees close to the geothermal plants almost systematically showed a lower Hg content in their rings with respect to trees located in the mining area. No significant variations were found in tree-rings corresponding to the years of AMIS installation (early 2000s). However, the Hg concentrations varied from 4 to 1.5 ng/g in reference area, indicating that trees are exposed to a modest local anomaly. All samples showed an increase in Hg concentration from hardwood to sapwood (up to an order of magnitude). Chestnut barks, that record the more present-day Hg pollution, systematically showed higher Hg concentrations than sapwood. This study shows that tree-rings may be a good record of Hg deposition in areas affected by mining activity and geothermal plants and can be used for impact minimization and optimal resource and land management. Nevertheless, further investigation of Hg cycling in trees is necessary to satisfactorily interpret this historical Hg record.
Mercury in chestnut tree-rings of the Monte Amiata area (Central Italy): impact of past mining activity and present-day geothermal power plants
Fornasaro S.
Primo
Membro del Collaboration Group
;
2022-01-01
Abstract
Mercury (Hg) is a global contaminant emitted to the atmosphere from both natural and anthropogenic sources, which have varied over time in response to increased industrialization and pollution control measures. The Monte Amiata Mining District (MAMD; Central Italy) hosts the 3rd largest Hg-district worldwide and an important geothermal field actively exploited for energy. Mining ceased in the 1980s but left an impressive legacy of environmental impact. The exploitation of geothermal energy started in 1959; currently, there are five active plants equipped with emission control systems (AMIS) to reduce Hg and H2S emissions; however, the efficiency of these systems is not 100% and Hg0 is still emitted from geothermal plants. Due to the Hg availability in the environment, plants are continuously exposed and absorb by resulting low-cost and efficient biological indicators. Specifically, trees store pollutants in tree-rings that have the potential for reconstructing past atmospheric Hg to complement the temporally and spatially limited data provided by the instrumental record, producing high-resolution data with absolutely dated chronologies. Here, we present a reconstruction of atmospheric Hg0 recorded in chestnut (Castanea sativa Mill.) tree-rings (1969−2020) from two different sites of the MAMD: i) site I near the former Abbadia SS mine; ii) site II near the main geothermal power plant. At each site three trees were sampled. A tree from the Appennino Pistoiese (150 km far from MAMD) (reference area) was analyzed for background value. Total Hg concentrations in the tree-rings were quantified with a Milestone tri-cell Direct Mercury Analyzer (DMA-80). The tree-rings in the MAMD provided a temporal reconstruction of Hg exposure at annual resolution. In the mining period Hg concentrations in trees from Abbadia SS varied between 244 and 50 ng/g, whereas in the post-production period after mine closure in the early 1980s, Hg showed a distinct decrease (between 113 and 12 ng/g). The trees close to the geothermal plants almost systematically showed a lower Hg content in their rings with respect to trees located in the mining area. No significant variations were found in tree-rings corresponding to the years of AMIS installation (early 2000s). However, the Hg concentrations varied from 4 to 1.5 ng/g in reference area, indicating that trees are exposed to a modest local anomaly. All samples showed an increase in Hg concentration from hardwood to sapwood (up to an order of magnitude). Chestnut barks, that record the more present-day Hg pollution, systematically showed higher Hg concentrations than sapwood. This study shows that tree-rings may be a good record of Hg deposition in areas affected by mining activity and geothermal plants and can be used for impact minimization and optimal resource and land management. Nevertheless, further investigation of Hg cycling in trees is necessary to satisfactorily interpret this historical Hg record.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
